Tuesday, September 6, 2016

Non-empirical confirmation stinks

String theory is a proof of the dangers of relying excessively on non-empirical arguments. It raised great expectations thirty years ago, when it promised to compute all the parameters of the Standard Model from first principles, to derive from first principles its symmetry group SU(3) x SU(2) x U(1) and the existence of its three families of elementary particles, to predict the sign and the value of the cosmological constant, to predict novel observable physics, to understand the ultimate fate of black holes and to offer a unique well-founded unified theory of everything. Nothing of this has come true. String theorists, instead, have predicted a negative cosmological constant, deviations from Newtons 1/r2 law at sub millimeters scale, black holes at CERN, low-energy supersymmetric particles, and more. All this was false.

From a Popperian point of view, these failures do not falsify the theory, because the theory is so flexible that it can be adjusted to escape failed predictions. But from a Bayesian point of view, each of these failures decreases the credibility in the theory, because a positive result would have increased it. The recent failure of the prediction of supersymmetric particles at LHC is the most fragrant [sic] example. By Bayesian standards, it lowers the degree of belief in string theory dramatically. This is an empirical argument. Still, Joe Polchinski, prominent string theorist, writes in [7] that he evaluates the probability of string to be correct at 98.5% (!).

I think he means the failure of SUSY is the most flagrant example. English is not his native language. Or maybe he means that string theory stinks!

At least string theory had some hope of doing something worthwhile, 20 years ago. There are other areas of theoretical physics that have no such hope.